This project is designed to quantitatively evaluate energy expenditure and the composition of the fuel mixture burned in patients with gastrointestinal cancer. In addition, glucose kinetics (oxidation rates, pool sizes, turnover rates and recycling) will be measured using 14C-U-L-glucose, the degree of gluconeogenesis will be measured using 14C-L-alanine, and whole body protein synthesis and catabolic rates will be measured using 15N-L-alanine. These kinetic studies will be carried out at the beginning of the patient's hospital stay and during nutritional support before and after surgical removal of the tumor. Nutritional support will be high fat-low carbohydrate or high carbohydrate-low fat diet with constant protein composition. The data should provide information on energy and substrate demands of the host and of the tumor by comparing the initial study (post-absorptive) with the pre-op study during nutritional support. Compartmentalization of the host to tumor demands may be ascertained by comparing the results before and after surgical removal of the tumor employing the same nutritional intake baseline in both studies. The postoperative study will be conducted only when the surgical catabolic transients are absent, which will be confirmed by a number of measured responses. It is hoped that these studies might provide additional information related to the amount and kind of nutritional support cancer patients should receive in their total care.